Hollow core optical fibre

ABSTRACT

A hollow core optical fibre comprises a tubular jacket; a cladding comprising a plurality of primary capillaries spaced apart from one another in a ring and each bonded to an inner surface of the jacket at a peripheral location around the circumference of the jacket; and a hollow core formed by a central void within the ring of primary capillaries; wherein the cladding further comprises, within each primary capillary, two secondary capillaries and no more, the two secondary capillaries spaced apart from one another and each bonded to an inner surface of the primary capillary at an azimuthal location around the circumference of the primary capillary which is displaced from the peripheral location of the primary capillary.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/266,685 filed Feb. 8, 2021, which is a national phase ofInternational Application No. PCT/GB2019/052145 filed Jul. 31, 2019,which claims priority to United Kingdom Application No. 1812909.8 filedAug. 8, 2018, all of which are hereby incorporated herewith by referencein their entireties.

BACKGROUND OF THE INVENTION

The present invention relates to hollow core optical fibres.

Classes of optical fibre include hollow core fibres, in which light isguided along a longitudinal hollow void forming the fibre's core by anoptical guidance mechanism enabled by the presence of a structuredarrangement of longitudinal cladding capillaries surrounding the void.Various configurations for the cladding capillaries are known, producingdifferent guidance mechanisms. An example is hollow core photonicbandgap fibre (HCPBF, alternatively hollow core photonic crystal fibres,HCPCF), in which the cladding comprises a regular closely packed arrayof many small glass capillaries from which a central group is excludedto define a substantially circular hollow core. The periodicity of thecladding capillary structure provides a periodically structuredrefractive index and hence a photonic bandgap effect that confines apropagating optical wave within the core.

In contrast to the microstructured array of cladding capillaries inHCPBF, a second type of hollow core fibre is the antiresonant hollowcore fibre, ARF. Fibres of this type have a simpler cladding geometry,comprising a lesser quantity of larger glass capillaries or tubesarranged in a ring around a central core void. The structure lacks anyhigh degree of periodicity so that no photonic bandgap exists, andinstead, antiresonance is provided for propagating wavelengths which arenot resonant with a wall thickness of the cladding capillaries, in otherwords, for wavelengths in an antiresonance window which is defined bythe cladding capillary wall thickness. The antiresonance acts to inhibitcoupling between air-guided optical modes supported by the core and anyoptical modes which the cladding may support, so that light is confinedto the core and can propagate at low loss by an antiresonant opticalguidance effect.

Modifications and variations to the basic ring of ARF claddingcapillaries have been proposed with the aim of enhancing fibreperformance. Many applications known for conventional solid core opticalfibres have been demonstrated with hollow core fibres, includingtelecommunications, optical power delivery and optical sensing. Fortelecommunications uses in particular, low optical loss (being thefraction of propagating light lost per unit length of propagation,typically per kilometre) is critical, and loss levels achieved thus farwith hollow core fibres are not yet fully competitive with theperformance available from solid core fibres.

Accordingly, designs of hollow core optical fibre offering improved lowoptical loss are of interest.

SUMMARY OF THE INVENTION

Aspects and embodiments are set out in the appended claims.

According to a first aspect of certain embodiments described herein,there is provided a hollow core optical fibre comprising: a tubularjacket; a cladding comprising a plurality of primary capillaries spacedapart from one another in a ring and each bonded to an inner surface ofthe jacket at a peripheral location around the circumference of thejacket; and a hollow core formed by a central void within the ring ofprimary capillaries; wherein the cladding further comprises, within eachprimary capillary, two secondary capillaries and no more, the twosecondary capillaries spaced apart from one another and each bonded toan inner surface of the primary capillary at an azimuthal locationaround the circumference of the primary capillary which is displacedfrom the peripheral location of the primary capillary.

According to a second aspect of certain embodiments described herein,there is provided a preform or a cane for making a hollow core opticalfibre which is configured to be drawn into a hollow core optical fibreaccording to the first aspect.

According to a third aspect of certain embodiments described herein,there is provided a preform or a cane for making a hollow core opticalfibre, and comprising: an outer tube for forming a jacket; a pluralityof primary tubes for forming primary capillaries to define a cladding ofthe fibre, the primary tubes spaced apart from one another in a ringaround a central void to form a hollow core, each primary tube arrangedagainst an inner surface of the outer tube at a peripheral locationaround the circumference of the outer tube; and within each primarytube, two secondary tubes and no more, the secondary tubes spaced apartfrom one another and each arranged against an inner surface of theprimary tube at an azimuthal location around the circumference of theprimary tube which is displaced from the peripheral location of theprimary tube.

These and further aspects of certain embodiments are set out in theappended independent and dependent claims. It will be appreciated thatfeatures of the dependent claims may be combined with each other andfeatures of the independent claims in combinations other than thoseexplicitly set out in the claims. Furthermore, the approach describedherein is not restricted to specific embodiments such as set out below,but includes and contemplates any appropriate combinations of featurespresented herein. For example, optical fibres may be provided inaccordance with approaches described herein which includes any one ormore of the various features described below as appropriate.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show how the same maybe carried into effect reference is now made by way of example to theaccompanying drawings in which:

FIG. 1(A) shows a schematic transverse cross-sectional view ofantiresonant hollow core fibres, including cladding features from aknown fibre design;

FIG. 1(B) shows a schematic transverse cross-sectional view ofantiresonant hollow core fibres, including cladding features fromanother known fibre design;

FIG. 2 shows a schematic transverse cross-sectional view of anantiresonant hollow core fibre of a known design;

FIG. 3 shows a schematic transverse cross-sectional view of anantiresonant hollow core fibre of a known design with superimposed linesto indicate transverse optical power flow derived from modelling;

FIG. 4 shows a schematic transverse cross-sectional view of a firstexample of an antiresonant hollow core fibre according to the presentdisclosure;

FIG. 5 shows a schematic transverse cross-sectional view of a secondexample of an antiresonant hollow core fibre according to the presentdisclosure;

FIG. 6 shows a schematic transverse cross-sectional view of an exampleantiresonant hollow core fibre according to the present disclosure withsuperimposed lines to indicate transverse optical power flow derivedfrom modelling;

FIG. 7 shows a graph of optical power loss per unit length (attenuation)against wavelength for the known fibre of FIG. 3 and the examples ofFIGS. 4 and 5 ;

FIG. 8 shows a partial transverse cross-sectional view of an examplefibre according to the present disclosure labelled with relevantdimensional and angular parameters;

FIGS. 9 and 10 show contour plots of optical attenuation for varyingcladding capillary spacings and angular positions modelled for examplefibres according to the present disclosure;

FIG. 11 shows a graph of optical attenuation against cladding capillarymisalignment angle modelled for example fibres according to the presentdisclosure;

FIG. 12 shows a partial transverse cross-sectional view of an examplefibre represented by data included in the graph of FIG. 11 ;

FIG. 13 shows a contour plot of optical attenuation for varying claddingcapillary sizes and angular positions modelled for example fibresaccording to the present disclosure;

FIG. 14A shows a schematic transverse cross-sectional view of a firstfurther example antiresonant hollow core fibres according to the presentdisclosure;

FIG. 14B shows a schematic transverse cross-sectional view of a secondfurther example antiresonant hollow core fibre according to the presentdisclosure;

FIG. 14C shows a schematic transverse cross-sectional view of a thirdfurther example antiresonant hollow core fibre according to the presentdisclosure;

FIG. 14D shows a schematic transverse cross-sectional view of a fourthfurther example antiresonant hollow core fibre according to the presentdisclosure; and

FIG. 14E shows a schematic transverse cross-sectional view of a fifthfurther example antiresonant hollow core fibre according to the presentdisclosure.

DETAILED DESCRIPTION

Aspects and features of certain examples and embodiments arediscussed/described herein. Some aspects and features of certainexamples and embodiments may be implemented conventionally and these arenot discussed/described in detail in the interests of brevity. It willthus be appreciated that aspects and features of optical fibresdiscussed herein which are not described in detail may be implemented inaccordance with any conventional techniques for implementing suchaspects and features.

The type of hollow core optical fibre that can be described asantiresonant hollow core fibre (ARF) at its simplest comprises a tubularouter jacket, and number of cladding capillaries arranged in a ringinside the jacket, and secured or bonded to the inner surface of thejacket. A central void within the ring of capillaries forms a hollowcore along which one or more optical modes can be guided by anantiresonant waveguiding effect.

FIG. 1(A) shows a transverse cross-sectional view of a firstpreviously-proposed antiresonant hollow core fibre. The view shows afull transverse cross section through a fibre with a circularcross-section. The fibre 10 has an outer tubular jacket 12. A pluralityof tubular or hollow cladding capillaries or cells 14, in this examplesix capillaries of the same cross-sectional size and shape, are arrangedinside the jacket 12 in a ring, so that the longitudinal axes of eachcladding capillary 14 and of the jacket 12 are substantially parallel.The cladding capillaries define elongate holes, lumen or cavities whichrun continuously along the length of the optical fibre. The number ofcapillaries allows this structure to be labelled as a 6 cell ARF. Thecladding capillaries or tubes 14 are each in contact with (bonded to)the inner surface of the jacket 12 at a location 16, such that thecladding capillaries 14 are evenly spaced around the inner circumferenceof the jacket 12, and are also spaced apart from each other (there is nocontact between neighbouring capillaries). The cladding structure islimited to these cladding capillaries only [1]. In some designs of ARF,the cladding tubes 14 may be positioned around the ring so that adjacenttubes are in contact with each other (in other words, not spaced apartas in FIG. 1(A)), but spacing to eliminate this contact can improve thefibre's optical performance. The spacing removes optical nodes thatarise at contact points between touching adjacent tubes and which tendto cause undesirable resonances that result in high losses. Accordingly,fibres with spaced-apart cladding capillaries as in FIG. 1(A) may bereferred to as “nodeless” antiresonant hollow core fibres.

The arrangement of the cladding capillaries 14 in a ring around theinside of the jacket 12 creates a central space, cavity or void withinthe fibre 10, also with its longitudinal axis parallel to those of thejacket 12 and capillaries 14, which is the fibre's hollow core 18, alsoextending continuously along the fibre's length. The core 18 is boundedby the inwardly facing parts of the outer surfaces of the claddingcapillaries 14. This is the core boundary, and the material (glass orpolymer, for example) of the capillary walls that make up this boundaryprovides the required antiresonance optical guidance effect ormechanism. The core boundary has a shape comprising a series of adjacentinwardly curving surfaces (that is, convex from the point of view of thecore). This contrasts with the usual outward curvature of thecore-cladding interface in a solid-core fibre, and the substantiallycircular core boundary of a hollow core photonic bandgap optical fibre.Accordingly, antiresonant hollow core fibres can be described asnegative curvature fibres. Mathematically, this can be defined as thesurface normal vector of the core boundary being oppositely directed toa radial unit vector (vector along a radius of the transversecross-section of the fibre). The negative curvature (convex shape) ofthe core boundary also inhibits coupling between the fundamental coremode and any cladding modes. A negative curvature antiresonant hollowcore fibre has a core boundary formed by a convex membrane or wall(typically of glass) which a thickness which is matched to be inantiresonance with the guided optical wavelength.

Some geometrical or dimensional parameters of interest are indicated inFIG. 1(A). The capillaries 14 have a wall thickness t. Each capillary 14is spaced apart from its neighbour by a gap or separation d, which isthe smallest distance between the outer surfaces of two adjacentcapillaries. Typically, the capillaries 14 are evenly spaced around theinner surface of the jacket 12, so each gap has the same value of d. Thecentral core 18 has a radius R, which is the smallest distance from thecentre of the fibre 10 (location of the fibre's longitudinal axis, whichis orthogonal to the plane of the page showing FIG. 1 ) to the outersurface of each cladding capillary 14. In this example, the capillariesare all the same size, so this distance is the same for each capillary14 and is the radius of the largest circle which can be fitted into thecross-section of the core 18.

FIG. 1(B) shows a transverse cross-sectional view of a secondpreviously-proposed antiresonant hollow core fibre [2, 3]. The fibreincludes all the features of the FIG. 1(A) example, but the cladding hasa more complex structure Each cladding capillary 14 is a primarycapillary, still spaced from its neighbour by a gap d, and has asecondary, smaller capillary 20 nested inside it, bonded to the innersurface of the cladding capillary 14 at the same azimuthal positionaround the jacket 12 as the point of bonding 16 between the primarycapillary 14 and the jacket 12. These additional smaller capillaries 20are included with the aim of reducing the optical loss in the fibre.Additional still smaller tertiary capillaries may be nested inside thesecondary capillaries, again bonded in line with the azimuthal contactlocations 16. ARF designs of this type, with secondary and optionallyfurther smaller capillaries, may be referred to as “nested antiresonantnodeless fibres”, or NANFs. The six primary capillaries of this exampleallow this structure to be labelled as a 6 cell NANF.

The example cladding structures shown in FIGS. 1(A) and 1(B) comprisesix primary cladding capillaries arranged in a ring around the core.ARFs are not so limited however, and may instead comprises five or feweror seven or more capillaries forming the boundary around the hollowcore. This is true of these previously-proposed examples, and of ARFsaccording to the present disclosure which are described below.

FIG. 2 shows a transverse cross-sectional view of another previouslyproposed antiresonant hollow core fibre structure [4]. In common withthe FIG. 1 example, the fibre comprises six primary capillaries 14bonded at positions 16 at regular intervals around the inner surface ofan outer tubular jacket 12, and spaced apart from one another. Insideeach primary capillary there are three secondary capillaries 20, eachbonded to the inner surface of the respective primary capillary 14. Ineach primary capillary, a first secondary capillary 20 a is bonded inline with the contact point 16 between the jacket 12 and the primarycapillary 14. The other two secondary capillaries 20 b are arranged oneon each side of the first secondary capillary 20 a and spaced apart fromthe first secondary capillary 20 a by an angle of 90°, measured as anazimuthal position around the circumference of the primary capillary 14.

It is commonly accepted in the technical field of hollow core opticalfibres that the inclusion of a secondary capillary nested inside aprimary capillary so that both capillaries are bonded at the sameazimuthal position on the circumference of the outer jacket (as in thelower half of FIG. 1 and FIG. 2 ) provides a second reflective element,in addition to that provided by the primary capillary, which acts toreduce optical loss. Furthermore it is also commonly assumed that thegap or spacing between the adjacent primary capillaries, while improvingperformance by removing unwanted resonances at contact between adjacentcapillaries, also contributes to the leakage of optical power.Therefore, the gap is generally made small to reduce leakage and henceloss, while being maintained above a zero spacing so as to avoidresonances.

However, the inventors have found that this theory of optical loss isnot correct, and based on an improved understanding of the lossmechanism in ARFs, propose an alternative configuration for the claddingcapillaries.

FIG. 3 shows a transverse cross-sectional view of part of an ARF with apreviously-proposed 6-cell NANF cladding structure (as described abovewith reference to FIG. 1(B)). Superimposed on it are black linesobtained from modelling the flow of optical power in the transverseplane, by tracing flow lines of the transverse Poynting vector. Opticalpower flowing radially outwards from the core to the outer surface ofthe cladding is power which is lost from the desired forward directionof propagation of power along the length of the fibre's core, and hencecontributes to the level of optical loss in the fibre. In FIG. 3 , theblack lines are streamlines that follow the transverse power flow vectorfields, which are seeded in the centre of the core and spiral outwardstowards the cladding tubes. The curve shown to the right of the fibreplots the total transversely leaked optical power, for each positionaround the circumference of the fibre (fibre boundary). The height ofthe curve measured from the depicted outer surface of the fibrecorresponds to the density of the power flow lines at the fibre-airboundary.

From this model, it is clear that the largest regions of power loss areat the locations of the nested cells. The radial region aligned with thebonding point 16 of the nested capillaries 14, 20 to the inside surfaceof the jacket 12 has the highest density of streamlines, and the curveof transverse power leakage has significant peaks at these locations(labelled “high density”). The adjacent regions on either side of eachbonding point 16, aligned with the roughly crescent-shaped regionsbetween the outer surface of a primary capillary 14 and the innersurface of the jacket 12 have a much lower transverse power leakage(labelled “low density”). The regions in the centre of each crescentregion which are radially aligned with the gaps between the spaced-apartprimary capillaries 14 have a slightly higher transverse power leakage(labelled “medium density”) which nevertheless is substantially lessthan the peak power loss through the nested capillary structure.

Therefore, the inventors have identified that the main source of radiallight leakage, which tends to be the dominant optical loss mechanism inARFs, is not the gaps between the spaced-apart primary capillaries. Thisis contrary to the general understanding of these fibres, in which thegaps are typically minimised to reduce loss. Rather, these parts of thecladding structure contribute only a small part in the overall loss, andinstead the principal loss of power arises at the regions in line withthe nested capillaries, at the azimuthal positions around the jacketwhere the secondary capillaries are bonded inside the primarycapillaries which are bonded inside the jacket. The transverse powerflow lines can be seen to have a complex behaviour, but it can bereadily appreciated that the streamlines exhibit their lowest density ineach gap between two adjacent primary capillaries 14, indicating lowtransverse power flow in these regions. As can be observed from FIG. 3 ,these features are the most effective parts of the cladding structure atlight confinement.

Accordingly, the present disclosure proposes a new geometry or design ofnested cladding capillaries for ARFs which introduces more of thenewly-identified leakage-reducing low loss features, while removing thehigh leakage features, namely the radial alignment of the nestedcapillaries. At the same time, the simplicity of known ARF designs ispreserved, together with the associated relative ease of fabrication.Structures in accordance with the disclosure show vastly superiorperformance in terms of reduced optical power loss.

FIG. 4 shows a transverse cross-sectional view of a first example hollowcore fibre according to the present disclosure. The fibre 30 comprisesan hollow tubular outer jacket 12. A series of six primary capillaries14 are arranged in a ring around the inner surface of the jacket 12. Theprimary capillaries 14 in this example each have the same size andcircular shape, and are each bonded, secured or fixed to the innersurface of the jacket 12 at a different azimuthal position 16 around thecircumference of the jacket 12 so that the primary capillaries 14 areevenly spaced around the ring. Also, the primary capillaries 14 are eachspaced apart from the two adjacent capillaries, so that there are gapsbetween adjacent capillaries, each gap being substantially the samewidth since the primary capillaries 14 have the same size and shape. Thecentral space or void defined by the surrounding ring of primarycapillaries 14 is the fibre's hollow core 18. The capillaries (alsotubes, cells or lumens) form the structured cladding of the fibre 30 forwaveguiding along the core 18. The core 18 has a radius R, being theradius of the largest circle that can be circumscribed around the core18 within the ring of primary capillaries 14, and hence also thedistance from the centre of the fibre's transverse structure to theclosest point on the outer surface of the primary capillaries 14.

Additionally, each primary capillary has two separate secondarycapillaries nested within it. By the term “nested” is meant that asecondary capillary is inside a primary capillary, with the longitudinaldimensions or axes of the two capillaries being substantially parallel,and also parallel to the longitudinal axes of the jacket and of thefibre overall. Also, each secondary capillary 20 is in contact with, andbonded, fixed or secured to, the inner surface of its associated primarycapillary 14 at a contact point or location 22, being an azimuthalposition around the circumference of the primary capillary. Within eachprimary capillary 14, the two secondary capillaries are sized andpositioned so that they are not in contact with one another. There is agap or space between the two secondary capillaries 20 which are insideeach primary capillary 14. This gap is the smallest separation betweenthe outer surfaces of the two secondary capillaries 20, in other wordsthe distance between the point on each outer surface which is closest tothe other outer surface. The secondary capillaries all have the samesize and circular shape. Significantly, neither secondary capillary 20is radially aligned within its primary capillary 14. The contact point22 at which each secondary capillary 20 is bonded to the primarycapillary 14 does not lie along a radius of the fibre 30 from the centreof the core 18 to the contact point 16 at which the primary capillary 14is bonded to the jacket 12. The contact point 22 at which each secondarycapillary 20 is bonded to the primary capillary 14 has an azimuthal orangular spacing, separation or displacement from the contact point 16 atwhich the primary capillary 14 is bonded to the jacket 12, around thecircumference (perimeter, periphery) of the primary capillary 14. Inthis particular example, the two secondary capillaries 20 in eachprimary capillary 14 have an equal and opposite azimuthal displacementfrom the contact point 16 of the primary capillary; they aresymmetrically positioned on either side of the radius from the corecentre (also the fibre centre) to the contact point 16 of the primarycapillary 14. Hence, a line joining the two contact points 22 of thesecondary capillaries 20 to the primary capillary 14 is orthogonal tothe radius from the core centre to the contact point 16 of the primarycapillary 14 to the jacket 12. The displacement in this example is 90°.Hence, the line joining the two contact points 22 lies along a diameterof the primary capillary 14 which is orthogonal to the radius from thecore centre to the contact point 16. Also, the radii of the primarycapillary which join the primary capillary centre to each of the contactpoints 22 lie along this same diameter.

FIG. 5 shows a transverse cross-sectional view of a second examplehollow core fibre according to the present disclosure. Whereas theexample of FIG. 4 has six evenly spaced primary capillaries, eachcontaining two secondary capillaries, the FIG. 5 fibre 30 has fiveprimary capillaries 14, again evenly spaced around the inner surface ofthe jacket, and of the same size and circular shape, and as before, eachcontaining two spaced apart secondary capillaries 20, each of the samesize and circular shape. Apart from this difference in the number ofprimary capillaries, the structure of the fibre 30 is the same as thatof the FIG. 4 example.

This arrangement of secondary capillaries within the primarycapillaries, away from the positions at which the primary capillariesare bonded inside the jacket, introduces into the cladding additionalshaped features which have been determined from the modelling shown inFIG. 3 to reduce radial leakage of optical power. As described above,the gaps between the outer surfaces of the adjacent primary capillariesshow the lowest radial power leakage. In FIG. 5 , one of these areas ishatched for illustrative purposes. Placement of two separate secondarycapillaries inside a primary capillary so that they are spaced apart oneither side of the azimuthal position of the primary capillary insidethe jacket creates an additional similarly shaped region or gap insideeach primary capillary. In FIG. 5 , one of these areas is cross-hatchedfor illustrative purposes. Thus, the low loss features are echoed andduplicated, and high loss features (i.e. the radial alignment of nestedcapillaries) are removed. This has the overall effect of providing anARF with superior loss characteristics, as will be described furtherbelow.

Note that each primary capillary has two, and only two (that is, not oneand not more than two, in other words more than one and fewer thanthree) secondary capillaries.

FIG. 6 shows a transverse cross-sectional view of part of an ARFaccording to the present disclosure and structured with six primarycapillaries as in the FIG. 4 example, with superimposed lines obtainedfrom modelling the flow of optical power in the transverse plane,similar to the modelling results shown in FIG. 3 for thepreviously-proposed 6-cell NANF cladding structure. The parameters usedfor the simulation are the same as those used to produce the resultsshown in FIG. 3 , the only difference being the cladding structure withtwo spaced-apart secondary capillaries 20 inside each primary capillary14. Again, on the right of the figure is shown a curve representing theamount of transverse power leakage. This curve is plotted on the samescale as the curve in FIG. 3 , as indicated by the dashed lines. Fromthis, it can be appreciated that the total power leakage is greatlyreduced by the new cladding geometry. Note that while the peaks of powerloss are still aligned with the contact points 16 where the primarycapillaries 14 are bonded to the jacket 12, the size of the peaks ismuch smaller than before, with the decreased loss attributable to thegap between the secondary capillaries 20 which lies along the sameradial position. The core region appears as solid black in this example;this is because the improved geometry reduces the total power loss awayfrom the core to such an extent that the spiralling streamlines have anextremely tight pitch and overlap one another. Note that, in contrast,the streamlines have a very low density in the gaps between theneighbouring primary capillaries 14 and in the newly added features ofthe gaps between the neighbouring secondary capillaries 20, indicatingthat power flow is confined away from these regions.

Although FIGS. 4 and 6 show an example with six primary capillaries orcells and FIG. 5 shows an example with five primary capillaries orcells, the disclosure is not limited in this regard. Fewer than fiveprimary cells may be used, for example four primary cells. More than sixprimary cells may be used, for example seven, eight, nine or ten primarycells, or more. Greater numbers of primary cells (such as more than six)may be useful in enabling a smaller overall fibre size when designing anARF for propagating longer wavelengths.

Returning to FIG. 5 , a region of width z is indicated, being a space orcavity in which a circle of diameter z is circumscribed inside theprimary capillary but outside the secondary capillaries, and opposite tothe contact point of the primary capillary with the jacket. The value ofz can be tuned for the suppression of higher order optical modes. Inorder to change the value of z (in other words, to change the diameterof the largest circle which can be circumscribed in the relevantcavity), the secondary capillaries can be varied in size and azimuthalposition around the primary capillary, and/or the size of the primarycapillary can be varied. The effect of varying these parameters isdiscussed in more detail below.

In the FIGS. 4 and 5 examples, the gaps between the two secondarycapillaries 20 in each primary capillary and the gaps between twoadjacent primary capillaries 14 are the same size. For the purpose ofmodelling the performance of these fibres (described further below), thegaps are selected to have a size of 5T, where T is the thickness of thewalls of the primary capillaries and the walls of the secondarycapillaries (selected in these examples to be the same). As exemplarydimensions, the core radius R may be 15 μm and the wall thickness t maybe 0.55 μm. These dimensions are suitable for a fibre designed forguiding light at a wavelength of substantially 1550 nm, which is astandard or common wavelength used in optical telecommunicationsapplications, owing to its minimal loss in silicon, from whichconventional solid core telecommunications optical fibre is made.Accordingly, the core radius is equal to about 10 wavelengths, and thecapillary wall thickness is equal to about 0.35 wavelengths. While thesevalues are for example only, the various simulations, models and datadiscussed below have been obtained for fibres with these dimensions. Theresults are presented with the geometric or dimensional parametersnormalised to wavelength, however, so that they can be scaled for use inthe design of fibres for guiding other wavelengths.

FIG. 7 shows a graph of modelled/simulated variation of optical loss(attenuation) with propagating wavelength for the two example fibres ofFIGS. 4 and 5 , compared to a NANF with six primary capillaries as inFIG. 3 [2] and the same dimensions for wall thickness and core size,propagating wavelength, and primary capillary spacing. The solid lineshows loss for the NANF, the dotted line shows loss for the five cellARF of FIG. 5 and the dashed line shows loss for the six cell ARF ofFIG. 4 . From this it can be appreciated that the optical performance ofthe proposed fibre design is excellent, and vastly improved compared tothe existing NANF. For the chosen core size, the NANF fibre cannotprovide loss performance which is 1 dB/km or less; its loss is greaterthan this for all wavelengths shown. However, for the newly-proposedfive cell fibre structure, the attenuation is significantly less thanfor the NANF across all wavelengths modelled, from 1.2 μm to 2.4 μm. Forthe newly-proposed six cell structure, the attenuation is below that ofthe NANF across the modelled wavelength range of 1.2 μm to about 1.9 μm.For wavelengths between about 1.25 μm and 2 μm, both examples of the newfibre offer attenuation below 1 dB/km, and the attenuation reducessubstantially around 1.6 μm. For the six cell fibre, there is an 85times improvement in optical confinement loss compared to the NANF. Thefive cell fibre performs even better. The reduced number of primarycapillaries allows for more space in the cladding structure (a largervalue of the width z can be achieved) which gives greater suppression ofhigher order optical modes, and superior confinement and reduced impactfrom resonances. The improvement in optical confinement loss compared tothe NANF is close to 450 times. Compared to the six cell fibre, the fivecell configuration is also easier to fabricate since it comprises fewerglass elements.

While FIG. 7 shows the loss reduction for the new structure comparedwith a NANF of the same core size, an alternative approach is tofabricate the new fibres to have a comparable loss to the NANF. Thisloss level is attainable at a smaller core size in the new fibres thanin the NANF, which offers the benefit of low loss compared to a simpleARF design such as FIG. 1(A) combined with a format that facilitatesinterconnection with conventional standard all-solid silica opticalfibres, which have a core size of R≈5 μm. This advantage is supplementedby a reduction in bend sensitivity, both macro and micro.

Other loss factors in optical fibres, such as surface scattering,macro-bending and micro-bending, have also been studied for the newdesign, and have been found to be comparable to the NANF structure asthese loss factors are attributable to the core size and shape which canbe the same as in the NANF structure.

For fabrication, the new design adds little complexity compared tofabrication of the existing NANF design. Fabrication of hollow corefibres with structured claddings formed from capillaries typicallyincludes pressurisation of the various spaces within the tubes toachieve and maintain the intended cross-sectional structure duringdrawing of the fibre from a preform or a cane, and the same number ofpressurisation regions or zones are needed for the new design as for theNANF. Compared to more complex nested geometries that comprise asmaller, tertiary capillary inside each secondary capillary, but whichare comparable to the new design in that two capillaries are nestedinside each primary capillary (a secondary and a tertiary instead ofnewly-proposed two secondaries), fabrication of the new design isconsiderably simpler since one fewer pressurisation zone is needed. Thenew design is also compatible with known hollow core fibre drawingtechniques, such as approaches using a glass working lathe to fuse theassembly of capillaries. [5]

FIG. 8 shows a transverse cross-sectional view of about one quarter ofan ARF configured in accordance with the present disclosure, on which anumber of geometric (dimensional and angular) parameters of interest areindicated. The primary capillaries 14 and the secondary capillaries 20each have a wall thickness T. The central hollow core of the fibre has adiameter R, being the distance from the central longitudinal axis of thefibre/jacket to the outer surface of the primary capillaries. Theprimary capillaries 14 have an internal radius r_(out), being thedistance from the central longitudinal axis of the primary capillary tothe inner surface of the primary capillary. The secondary capillaries 20have an internal radius r_(in), being the distance from the centrallongitudinal axis of the secondary capillary to the inner surface of theprimary capillary. The primary capillaries 14 are substantially evenlypositioned around the jacket circumference, with a spacing d_(out)between each adjacent pair of primary capillaries, being the distancebetween the two closest points of the outer surfaces of the adjacentprimary capillaries. Within each primary capillary 14, the two secondarycapillaries 20 are spaced apart by a spacing d_(nest), being thedistance between the two closest points on the outer surfaces of thesecondary capillaries. Each primary capillary 14 is bonded to the jacket12 at a contact point 16, each contact point 16 at a different azimuthalposition around the circumference or periphery of the jacket 12, andregularly spaced. Within each primary capillary 14, the two secondarycapillaries 20 are each bonded to the inner surface of the primarycapillary at a contact point 22, the two contact points at a differentazimuthal position around the circumference of the outer capillary 14.Neither contact point 22 is coincident with the contact point 16 betweenthe primary capillary 14 and the jacket 12. For the purposes ofdistinction, the contact point 16 of each primary capillary can beconsidered to have a peripheral location or position around the jacket,and the contact point 22 of each secondary capillary can be consideredto have an azimuthal location or position around the primary capillary,where the azimuthal locations are each displaced or separated from theperipheral location, in other words, neither azimuthal positioncoincides with the peripheral location. A diameter of each primarycapillary 14 lies along a line of mirror symmetry 24 between the twosecondary capillaries, the line 24 meeting the wall of the primarycapillary 14 closest to the jacket at a point 26. Hence the point 26 isa midpoint between the two contact points 22, equidistant from eachcontact point 22. The two contact points 22 for the two secondarycapillaries have an angular displacement θ from the line of mirrorsymmetry 24, measured as an azimuthal displacement around thecircumference of the primary capillary from the point 26. Since the line24 is a line of mirror symmetry between the two secondary capillaries,the displacements θ of the secondary capillaries are equal and opposite.The midpoint 26 between the secondary capillaries 20 has an angular orazimuthal displacement ϕ from the contact point 16, measured around thecircumference of the primary capillary 14. These various parameters canbe varied to adjust the optical loss of the fibre.

Modelling of the performance of fibres configured in accordance with thepresent disclosure enables the identification of ranges for the variousgeometric parameters which are able to deliver particularly goodperformance as regards optical loss of a fibre. As noted above,simulations have been carried out using representative dimensions whichare appropriate for waveguiding of the standard telecommunicationswavelength of 1550 nm, namely a core radius R of 15 μm and a capillarywall thickness T of 0.55 μm, with the geometric dimensions thennormalised to wavelength to allow scaling for the fabrication of fibresto guide alternate wavelengths of light.

The radius of the primary capillaries 14, r_(out), is given by

$r_{out} = \frac{{\left( {R + T} \right){\sin\left( \frac{\pi}{n} \right)}} - T - \frac{d_{out}}{2}}{1 - {\sin\left( \frac{\pi}{n} \right)}}$where the parameters are as defined above with reference to FIG. 7 , andn is the number of primary capillaries. For the results of simulationsdiscussed below, n is five, but the results are applicable to othernumbers of the primary capillaries.

The radius of the secondary capillaries 20, r_(in), is given by

$r_{in} = \frac{{\left( {r_{out} - T} \right)\;\sin\;\left( \frac{\pi}{n} \right)} - \left( {\frac{d_{nest}}{2} + T} \right)}{1 + {\sin\;\theta}}$with the parameters defined as before.

FIG. 9 shows a contour plot of optical confinement loss (dB/km) for afibre with characteristics as described above, showing the behaviour ofthe loss as the gap d_(nest) between the secondary capillaries(normalised to the wavelength λ), and the angle θ by which the secondarycapillaries are displaced from the central position are varied. Theangle ϕ is set to zero for this modelling, so the secondary capillariesare symmetrically positioned on either side of the contact point wherethe primary capillary is bonded to the jacket. The grey region shows therange of the parameters which give confinement loss of less than 1dB/km. The smaller black region shows the range of the parameters whichgive confinement loss of less than 0.01 dB/km. From this, it can beappreciated that values of θ at or near to 90° (as depicted in FIGS. 4and 5 ) are particularly beneficial in achieving very low loss. Forexample, θ may be substantially 90°, or may be in the range of 85° to95°

FIG. 10 shows a contour plot of optical confinement loss (dB/km)obtained from the same modelling as for FIG. 8 , but showing thebehaviour of the loss as the gap d_(out) between the primary capillaries(normalised to wavelength λ) and the gap d_(nest) between the secondarycapillaries (normalised to the wavelength λ) are varied. The angle ϕ isset to zero as before. The grey region shows the range of the parameterswhich give confinement loss of less than 1 dB/km. Recall that thisperformance cannot be achieved from the previously-proposed NANFstructure. The smaller black region shows the range of the parameterswhich give confinement loss of less than 0.01 dB/km.

From FIGS. 9 and 10 , it can be deduced that to achieve confinement lossof 1 dB/km or below, the angle θ has a value in the range of 30° to142°, the gap or spacing between the primary capillaries (normalised towavelength) d_(out)/λ has a value in the range of −0.3 to 4.5, and thegap or spacing between the secondary capillaries (normalised towavelength d_(nest)/λ has a value in the range of −0.3 to 6.7. Note thatnegative values of the spacings indicates that contact between thecapillaries has occurred. Since contact is associated with largerresonances that cause increased loss in the optical transmission windowfor the fibre, it is possible that fibres with parameters in these partsof the ranges may show larger loss than the intended maximum value of 1dB/km. Accordingly, the ranges may be preferred to be >0 to 4.5 ford_(out)/λ and >0 to 6.7 for d_(nest)/λ. The specification of the lowerlimits of theses range as “>0” indicates that the capillaries should bearranged so as not to be in contact, i.e. the gap or spacing betweenthem has a finite positive value.

Similarly, to achieve confinement loss of 0.01 dB/km or below ranges canbe deduced from FIGS. 9 and 10 as follows: the angle θ has a value inthe range of 75° to 112°, d_(out)/λ has a value in the range of >0 to3.1 and d_(nest)/λ has a value in the range of >0 to 3.4.

As remarked, the modelling for FIGS. 9 and 10 assumes a value of theangle ϕ of 0°. In other words, the nested element or arrangement of aprimary capillary with two secondary capillaries is radially aligned inthat the midpoint or line of symmetry between the azimuthal positions ofthe secondary capillaries within the primary capillary is coincidentwith the contact point of the primary capillary within the jacket. Thesecondary capillaries have equal and opposite angular displacements fromthis contact point; they are positioned symmetrically. In reality, thispositioning may be difficult to achieve since some shifting or rotationof the nested elements with respect to their intended positons may occurduring fabrication of a preform or drawing of the fibre from thepreform. Accordingly, it is important to consider the effect ofmisalignment of the secondary capillaries from the symmetricalpositioning.

Recall that in FIG. 8 , we defined the angle ϕ as being the angulardisplacement, around the circumference of the primary capillary, of themidpoint between the contact points of the two secondary capillariesfrom the contact point of the primary capillary with the jacket. Anon-zero value of ϕ can be considered to be a misalignment of the nestedarrangement. Modelling of the confinement loss has been carried out toinvestigate the effect of misalignment.

FIG. 11 shows a graph of the amount of confinement loss in dB/km as themisalignment varies over the range of 0° to 90° for ϕ. For themodelling, the angular position θ of the secondary capillaries was setto be 90°, for minimal loss in line with the results shown in FIG. 9 .The gaps d_(out)/λ and d_(nest)/λ had values of 1 and 2 respectively. Ascan be appreciated, for misalignment angles up to about 30°, there is noappreciable change in the loss, which remains well below 0.01 dB/km.This is a very useful result, indicating that a moderate level ofmisalignment can be tolerated in a fibre without impacting on the lossperformance. Thus, manufacturing tolerances for the nested elementalignment need not be overly constrained. Beyond 30°, the loss begins toincrease. Interestingly, for secondary capillaries positioned at θ=90°,this corresponds with the contact point 22 of a secondary capillary 20on the primary capillary 14 approaching the location of the gap d_(out)between the neighbouring primary capillaries. The positions of thecontact point and the gap coincide when ϕ=36°, for θ=90°. Thisconfiguration is shown in FIG. 12 .

Beyond this position, as the contact point 22 passes the location of thenarrowest part of the gap between the primary capillaries and movestowards the core so that the contact point 22 becomes part of the coreboundary, the loss increases rapidly, as can be seen from FIG. 10 . Amisalignment angle of 38° corresponds to a loss of 0.01 dB/km, marked inFIG. 11 by a dot-dash line. A misalignment angle of 47° corresponds to aloss of 0.1 dB/km, marked in FIG. 11 by a dashed line. A misalignmentangle of 53° corresponds to a loss of 1 dB/km, marked in FIG. 11 by adotted line. Accordingly, in order to achieve a confinement loss levelof 1 dB/km or below, the misalignment angle ϕ should be in the range of0° to 53°, and to achieve a confinement loss level of 0.01 dB/km orbelow, the misalignment angle ϕ should be in the range of 0° to 38°. Foran intermediate loss level of 0.1 dB/km or below, ϕ should be in therange of 0° to 47°. Note that the values of ϕ are modular values, so therotation of the nested arrangement away from ϕ=0° may be in eitherdirection; it is not restricted to rotation in the anticlockwisedirection suggested in FIG. 12 .

Note that the parameters of azimuthal position θ and capillary spacingsd_(out)/λ and d_(nest)/λ map onto the sizes of the capillaries, whichvary to accommodate changes in the angle and spacings. Accordingly, theachievable loss properties available from the new fibre designs mayalternatively or additionally be defined by reference to the capillarysizes. In particular, one can consider the ratio of the sizes of theprimary and secondary capillaries, which has a limit set by the need toaccommodate two secondary capillaries in a non-contacting arrangementwithin a primary capillary.

FIG. 13 shows a contour plot of optical confinement loss similar to theplots of FIGS. 9 and 10 (and obtained by reformulation of the data shownin FIGS. 9 and 10 ), but showing the loss achievable as both theazimuthal position θ of the secondary capillaries and the ratio of thesecondary capillary radius to the primary capillary radiusr_(in)/r_(out) are varied. As before, the grey area indicates loss of 1dB/km or below and the black area indicates loss of 0.01 dB/km.

From FIG. 13 , we therefore deduce that to provide attenuation of 1 dBor below, the secondary capillaries should have a radius r_(in) in therange of 0.29 r_(out), the radius of the primary capillary, or above.For an attenuation of 0.01 dB/km the secondary capillaries should belarge, with a radius r_(in) in the range of 0.38 r_(out) or above. Inboth cases, this defines a lower limit or minimum size for the secondarycapillaries. The maximum permissible size for the secondary capillariesis limited by the need to provide a finite gap, d_(nest), between thetwo secondary capillaries. From FIG. 13 , this can be seen to be about0.47 r_(out) for both attenuation levels. These ranges correspond to therange for the angle θ of about 30° to 142° also evident from FIG. 9 .

While the plots of FIGS. 9, 10 and 13 show contours for loss levels of 1dB/km and below and 0.01 dB/km and below, it is possible to also defineranges for the various geometric parameters that correspond to otherloss levels. Table 1 shows ranges defined in terms of upper and lowerlimits for the angle θ and the spacings d_(out)/λ and d_(nest)/λ forloss levels of 1 dB/km (as already given above), 0.2 dB/km, 0.1 dB/kmand 0.01 dB/km (also as already given above). A loss value of 0.2 dB/kmis the telecommunications industry standard for conventional silicasolid core optical fibre.

TABLE 1 Para- 1 dB/km 0.2 dB/km 0.1 dB/km 0.01 dB/km meter Lower UpperLower Upper Lower Upper Lower Upper θ° 30 142 48 123 50 150 75 112d_(out)/λ −0.3 4.5 −0.2 4.1 −0.2 3.9 >0 3.1 d_(nest)λ −0.3 6.7 −0.3 5.55−0.3 5.1 >0 3.4

Accordingly, an antiresonant hollow core optical fibre with a claddingstructure configured in accordance with the present disclosure offerssignificantly improved performance as regards propagation loss comparedwith existing hollow core optical fibres designs. Such fibre is suitablefor a wide range of optical fibre applications, in particular opticalfibre telecommunications,

A hollow core fibre in line with the present disclosure can befabricated using known methods for making antiresonant hollow corefibres, which can be drawn in a conventional manner from a preform,optionally via an intermediate cane, configured with the transversecross-sectional structure desired for the finished fibre but on a largerscale. The known reduction in cross-sectional area from a preform to afinished fibre can be used to appropriately scale up the dimensions setout herein to construct, from suitably sized tubes or tubular elements,preforms from which fibres dimensioned according to the presentdisclosure can be fabricated. Similarly, the fibres may be made frommaterials known for the fabrication of existing designs of antiresonanthollow core fibre, glass materials such as silica, and polymermaterials. The various tubes or capillaries (outer jacket and primaryand secondary capillaries) in a single preform or fibre may be made fromthe same material or from different materials. Types of glass include“silicate glasses” or “silica-based glasses”, based on the chemicalcompound silica (silicon dioxide, or quartz), of which there are manyexamples. Other glasses suitable for optical applications include, butare not limited to, chalcogenide, tellurite glasses, fluoride glasses,and doped silica glasses. The materials may include one or more dopantsfor the purpose of tailoring the optical properties, such as modifyingabsorption/transmission or enabling optical pumping.

The ARF cladding structure disclosed herein is not limited to theexamples given, and an antiresonant hollow core optical fibre inaccordance with the present disclosure may have a modified structurecompared to these examples, and/or may include additional features orelements in the cladding.

FIG. 14 shows a selection of such ARFs, with structures comprisingmodifications or additional features compared to the examples describedthus far. FIG. 14A shows a cross-sectional transverse view of an exampleantiresonant hollow core optical fibre in which the jacket 12 has asubstantially square cross-section, with four primary capillaries 14bonded to its inner surface, one primary capillary 14 at each corner ofthe square shape. Each primary capillary 14 has two spaced-apartsecondary capillaries 20 positioned within, as described herein.

FIG. 14B shows a cross-sectional transverse view of an exampleantiresonant hollow core optical fibre which comprises six evenly-spacedapart primary capillaries 14 each with two secondary capillaries 20inside, as in the FIG. 4 example (although with a smaller value of theangle θ). Additionally, each secondary capillary 20 has two smallertertiary capillaries 28 bonded to its inner surface in a spaced-apartarrangement, to create further low loss gap features (between theadjacent tertiary capillaries 28 inside each secondary capillary 20)echoing the gaps between the primary capillaries 14.

FIG. 14C shows a cross-sectional transverse view of an exampleantiresonant hollow core optical fibre again comprising six-evenlyspaced apart primary capillaries 14 each with two secondary capillaries20 as described with regard to FIG. 4 , and also having smaller tertiarycapillaries 28 nested inside the secondary capillaries 20. Unlike theFIG. 14B example, in this case there is only one tertiary capillary 28inside each secondary capillary 20. The tertiary capillary 28 is bondedto the inner surface of the secondary capillary 20 in line with the bondposition 22 of the secondary capillary 20 on the inner surface of theprimary capillary 14.

FIGS. 14D and 14E each show a cross-sectional transverse view of anexample antiresonant hollow core optical fibre, which, in common withthe two preceding examples, has six evenly spaced apart primarycapillaries 14 inside the jacket 12, each of which has two secondarycapillaries 20 positioned in accordance with the present disclosure. Notertiary capillaries are included. Rather, these examples includes sixadditional tubes or capillaries 32, smaller than the primary capillaries14, and positioned against the inner surface of the jacket 12 betweenthe primary capillaries 14. In the FIG. 14D example, there are gapsbetween the additional capillaries 32 and the adjacent primarycapillaries 14, while in the FIG. 14E example, there are no such gaps.

Other cladding structures are not excluded, and are considered withinthe scope of the present disclosure to the extent that they include twosecondary capillaries within each primary capillary and positioned asdescribed herein, namely such that they are spaced apart from oneanother and each bonded to an inner surface of the primary capillary atan azimuthal location around the circumference of the primary capillarywhich is displaced from the peripheral location of the primary capillarywithin the jacket of the hollow core optical fibre.

Preforms and canes for making hollow core optical fibres as describedherein are also contemplated, where a preform or a cane comprises acollection of tubes each corresponding to a capillary or jacket of theintended optical fibre structure and positioned relative to the othertubes so as to produce a hollow core optical fibre with a jacket,cladding and hollow core as described herein.

The various embodiments described herein are presented only to assist inunderstanding and teaching the claimed features. These embodiments areprovided as a representative sample of embodiments only, and are notexhaustive and/or exclusive. It is to be understood that advantages,embodiments, examples, functions, features, structures, and/or otheraspects described herein are not to be considered limitations on thescope of the invention as defined by the claims or limitations onequivalents to the claims, and that other embodiments may be utilisedand modifications may be made without departing from the scope of theclaimed invention. Various embodiments of the invention may suitablycomprise, consist of, or consist essentially of, appropriatecombinations of the disclosed elements, components, features, parts,steps, means, etc., other than those specifically described herein. Inaddition, this disclosure may include other inventions not presentlyclaimed, but which may be claimed in the future.

REFERENCES

-   [1] Anton N. Kolyadin, Alexey F. Kosolapov, Andrey D. Pryamikov,    Alexander S. Biriukov, Victor G. Plotnichenko, and Evgeny M. Dianov,    “Light transmission in negative curvature hollow core fiber in    extremely high material loss region”, Opt. Express 21, 9514-9519    (2013)-   [2] Francesco Poletti, “Nested antiresonant nodeless hollow core    fiber”, Opt. Express 22, 23807-23828 (2014)-   [3] WO 2015/185761-   [4] Md. Selim Habib, Ole Bang, and Morten Bache, “Low-loss    hollow-core silica fibers with adjacent nested anti-resonant tubes”,    Opt. Express 23, 17394-17406 (2015)-   [5] A F Kosolapov, G K Alagashev, A N Kolyadin, A D Pryamikov, A S    Biryukov, I A Bufetov and E M Dianov, “Hollow-core revolver fibre    with a double-capillary reflective cladding”, Quantum Electronics,    46(3), 267 (2016)

The invention claimed is:
 1. A hollow core optical fibre comprising: atubular jacket; a cladding comprising a plurality of primary cellsspaced apart from one another in a ring around an inner surface of thejacket with gaps between adjacent cells; and a hollow core formed by acentral void within the ring of primary cells; wherein the claddingfurther comprises, inside each primary cell, two secondary cells and nomore, the two secondary cells spaced apart from one another and a gapbetween the two secondary cells, neither secondary cell being radiallyaligned with the primary cell along a radius of the hollow core opticalfibre.
 2. A hollow core optical fibre according to claim 1, in which,inside each primary cell, the secondary cells are spaced apart on eitherside of an azimuthal position of the primary cell inside the jacket. 3.A hollow core optical fibre according to claim 2, in which eachsecondary cell has a contact point around the perimeter of the primarycell which has a displacement from the azimuthal position of the primarycell inside the jacket.
 4. A hollow core optical fibre according toclaim 2, in which each secondary cell has a contact point around theperimeter of the primary cell which has a displacement from a contactpoint of the primary cell with the inner surface of the jacket.
 5. Ahollow core optical fibre according to claim 2, in which each secondarycell has a contact point around a perimeter of the primary cell which isnot coincident with a contact point of the primary cell with the innersurface of the jacket.
 6. A hollow core optical fibre according to claim2, in which the secondary cells have equal and opposite displacementsfrom the azimuthal position of the primary cell inside the jacket.
 7. Ahollow core optical fibre according to claim 2, in which the secondarycells are misaligned from having equal and opposite displacements fromthe azimuthal position of the primary cell inside the jacket.
 8. Ahollow core optical fibre according to claim 1, comprising four primarycells.
 9. A hollow core optical fibre according to claim 1, comprisingfive primary cells.
 10. A hollow core optical fibre according to claim1, in which the primary cells are spaced apart by gaps d_(out) in therange of 0<d_(out)/λ≤4.5 and the secondary cells are spaced apart bygaps d_(in) in the range of 0<d_(in)/λ≤6.7, where λ is an opticalwavelength of light which the hollow core optical fibre is configured toguide.
 11. A hollow core optical fibre according to claim 1, in whichthe primary cells are spaced apart by gaps d_(out) in the range of0<d_(out)/λ≤3.1 and the secondary cells are spaced apart by gaps d_(in)in the range of 0<d_(in)/λ≤3.4, where λ is an optical wavelength oflight which the hollow core optical fibre is configured to guide.
 12. Ahollow core optical fibre according to claim 1, and having a level ofoptical propagation loss for guided light at a wavelength which thehollow core optical fibre is configured to guide which is 1 dB/km orless.
 13. A hollow core optical fibre according to claim 1, and having alevel of optical propagation loss for guided light at a wavelength whichthe hollow core optical fibre is configured to guide which is 0.01 dB/kmor less.
 14. A preform or a cane for making a hollow core optical fibrewhich is configured to be drawn into a hollow core optical fibreaccording to claim 1.